Sponge consists of a reticulated fibrous mass, covered with numerous pores of various sizes, which are connected internally by anastomosing channels, and this tissue is surrounded by a cellular gelatinous matter, by which the entire structure was secreted, and is, in fact, the vital part of the zoophyte. The tough framework or skeleton is in some kinds fibrous, horny, flexible, or rigid, and strengthened by calcareous or siliceous spicula (_spines_);[194] while in other species its substance is calcareous, and in some siliceous, constituting a web of transparent rock crystal, resembling spun glass,[195] The gelatinous matter lines all the cavities, and forms the margins of the openings; it presents no signs of irritability, and may be easily pressed out of the porous mass with the hand, so slight is the connexion between the skeleton and the investing tissue. Currents of water constantly enter the small pores, traverse the inosculating canals, and issue from the larger orifices, which often project above the surface in perforated papillæ. By the circulation of the water through the porous structure, the nutrition of the organized mass is effected; and the modifications observable in the number, size, form, and disposition of the pores, channels, and orifices, in different species, appear to be subservient to this especial purpose; the imbibition and expulsion of water being the only function the sponges perform. In its earliest stage the sponge gemmule is of a spheroidal shape, and covered with vibratile cilia, and after expulsion from the canals in which it is formed, moves rapidly through the water till it becomes attached to some body, and is then immovably fixed during life; exhibiting no signs of vitality save the aqueous circulation through the pores and canals. [194] The Mediterranean and American sponges of commerce are devoid of spicules, and are deprived of their soft animal matter simply by washing freely in fresh water. [195] I particularly allude to a siliceous Sponge from Barbadoes, named, by Mr. Samuel Stutchbury, formerly of the Bristol Institution, (now of Australia.) _Dictyochalix pumicea_. This specimen is of a fungiform shape, and appears to the naked eye as if formed of pumice stone, but under the microscope is literally a tissue of transparent silex. There is much confusion in the arrangement and nomenclature of the fossil species of this class of zoophytes; and this has originated, in part, from the varied forms assumed by the same species, having been described under different names; and from the reprehensible practice of changing, without sufficient reason, the name assigned to a species by the original discoverer; an evil, unfortunately, not restricted to this department of natural history. The recent Sponges are arranged in four groups according to their structure, viz.-- _Fresh-water Sponges._ Spongilla: siliceous spicula in a translucent jelly-like mass. _Marine Sponges._